# from: from: https://serverfault.com/questions/922248/how-to-configure-postfix-behind-haproxy 
# Exposed SMTP service (postscreen support is needed to support the proxy protocol [search postscreen_upstream_proxy_protocol in main.cf])
smtp      inet  n       -       -       -       1       postscreen
smtpd     pass  -       -       -       -       -       smtpd